(1) Field of the Invention
This invention relates to a gear shift control apparatus for a hybrid vehicle which includes a motor and an engine.
(2) Description of the Related Art
Generally, an automatic transmission incorporated in an automobile includes a gear shift map in which the vehicle speed and the accelerator opening are used as parameters. Usually, a target gear position is set based on the gear shift map, and the gear position is changed over to the target gear position. On the other hand, in a manual transmission, the driver operates a shift lever of its own will to changeover the gear position.
A technique relating to a gear shift control apparatus for an automatic transmission is disclosed in Japanese Patent Laid-Open No. Hei 5-26335 (hereinafter referred to as Patent Document 1). In particular, according to the technique disclosed in Patent Document 1, an automatic transmission which includes a main transmission section and a sub transmission section and wherein the gear position is changed over such that a low speed gear position is assumed when the sub transmission section is in a directly coupled state whereas a high speed gear position is assumed when the sub transmission section is not in a directly coupled state is configured such that, upon down shifting, the sub transmission section assumes the gear position of the directly coupled state.
According to the technique, the ratio of a period of time within which the sub transmission section assumes the directly coupled state upon down shifting is increased by such control as described above to achieve enhancement of the fuel cost and reduction of gear noise and further achieve enhancement of the durability.
Incidentally, in a transmission, a gear position (directly coupled gear position) at which the transmission gear ratio (output rotational speed/input rotational speed) is 1.0 is the gear position at which the highest transmission efficiency is obtained. Meanwhile, in a transmission which does not have a directly coupled gear position, a gear position at which the transmission gear ratio is nearest to 1.0 is the gear position at which the highest transmission efficiency is obtained. The transmission gear position at which the transmission efficiency is highest provides an advantage that an enhanced fuel cost is obtained when compared with the other gear positions.
However, conventional popular automatic transmissions are not configured taking such a situation as described above into consideration, but a gear shift map is set based on a torque characteristic of a driving source (engine). Accordingly, in conventional popular automatic transmissions, if the engine operation condition is changed to a region of another gear position on the gear shift map, then gear shifting is performed even if traveling at the directly coupled gear position is preferable from the fuel cost performance.
Further, in the technique disclosed in Patent Document 1, while, upon down shifting, the subtransmission section is shifted to the gear position of the directly coupled state, the target gear position of the main transmission section is set based on an ordinary gear shift map. In this regard, the technique disclosed in Patent Document 1 is a similar technique to the popular automatic transmissions described hereinabove and does not take the transmission efficiency at the main transmission section into consideration. Further, with the technique disclosed in Patent Document 1, since the target gear position is likely to be set to a gear position different from an original target gear position, there is the possibility that the drivability may be damaged.